This project aims at describing specific developmental processes in calcified tissues at the molecular level. It comprises various lines of multidisciplinary research that will: (1) study specific structural characteristics --including the defect structure-- of single crystals and polycrystalline materials of hydroxyapatite and chlorapatite, using x-ray diffraction, EPR optical spectroscopy and other physical methods; and the resulting specific properties; (2) study specific structural characteristics of molecular packing in collagen fibrils from various tissues -- soft and calcified -- and the resulting specific properties; (3) study the kinetics of collagen metabolism in various tissues -- soft and calcified -- and, specifically, the fate of newly formed collagen molecules in relation to tissue organization; (4) study in vivo and in vitro the growth and mineralization of bones and teeth. These research objectives are complementary to those of a detailed study of cytodifferentation in bone -- using, in particular, isolated bone cells -- carried out by another research team in the same department.